Former Defense Secretary Dr. James Schlesinger in 2004. (Rob Curtis/Staff)

Former Defense Secretary James Schlesinger has died, according to the Center for Strategic and International Studies, where he served as a counselor and trustee. He was 85.

Schlesinger served as secretary of defense from 1973 until 1975 under Presidents Richard Nixon and Gerald Ford. When he left the Pentagon, Donald Rumsfeld took his place.

“Joyce and I were deeply saddened to learn of the death of our friend, Jim Schlesinger,” Rumsfeld said in a statement on Thursday. “Jim was a dedicated public servant, a skilled economist, and an American patriot who served in senior positions in three presidential administrations. He was a frequent source of wise counsel to me. For his insight, he won respect from Republicans and Democrats alike. We offer our deepest sympathies to his family and share the loss that I know is deeply felt among his many friends, colleagues, and admirers.”

Marine veteran and author Bing West remembers Schlesinger as a friend and a mentor who helped the Marine Corps rebuild after Vietnam. In 1975, he was so disturbed by the condition of the Marine Corps that he met with several general officers including then Lt. Gen. Louis Wilson Jr., a Medal of Honor recipient.

“Schlesinger digested what they told him and on a Saturday morning, drove to Sen. [John ] Stennis’ home, the Chairman of the Armed Services Committee,” West wrote in an email Thursday to Military Times. “He then called Gen. Wilson to tell him he would be the next Commandant, with permission to put [then Lt. Gen. Robert] Barrow in charge of manpower and full authority from Stennis and Schlesinger to take whatever steps he believed were necessary to rebuild the Corps.”

During his tenure as defense secretary, Schlesinger was also very close to former Army Chief of Staff Gen. Creighton Abrams, the legendary commander of U.S. forces in Vietnam and namesake for the Abrams tank, West said. One night, Abrams invited Schlesinger over for dinner.

“Abe lit a cigar, sipped his brandy and said, ‘I’m dying of cancer, Jim. We’d best get to work selecting my successor. Doc says I can drink and smoke as much as I want. Won’t even allow me to wallow in any self-pity. So let’s have another drink and another cigar,’” West said.

Years after leaving the Pentagon, Schlesinger was called back to service in June 2008 when then Defense Secretary Robert Gates tasked him with leading an investigation into the security and control of the nation’s nuclear arsenal after Gates fired Air Force Secretary Michael W. Wynne and Chief of Staff Gen. T. Michael Moseley over incidents in which the Air Force mishandled nuclear weapons.

In September 2008, Schlesinger told reporters that the initial findings of the investigation showed that the Air Force’s focus on the nuclear mission had been underfunded and understaffed after Strategic Air Command was disestablished in 1991.

The task force would recommend that all nuclear forces be placed under one major command. The following month, the Air Force announced the creation of Global Strike Command to do precisely that.

“We are not just interested in what the capabilities are today,” Schlesinger said at a Jan. 8, 2009 news conference announcing the investigation’s full findings. “We are concerned, as we look out five, 10 years, that to the extent that we need a nuclear deterrent — and the Commission on Strategic Posture appointed by the Congress states that we will need a nuclear deterrent for the indefinite future — that there be no doubt in the minds of any observers in foreign capitals as to the strength, the credibility — indeed, the impressiveness — of the nuclear deterrent of the United States.”

On Thursday, Gates issued statement saying he will miss Schlesinger, whom he called his “friend and mentor.”

“With the passing of Dr. James Schlesinger, the United States has lost a great patriot and one of our finest minds on defense and national security,” Gates said in the statement. “He served both Republican and Democratic presidents in senior positions with equal zeal and skill. He was a source of good counsel and extraordinary insight for multiple secretaries of defense and heads of American intelligence.”

Schelsinger was appointed director of the CIA by President Nixon in February 1973, but served in that role for only a few months before moving to the Pentagon to take over as defense secretary in July 1973.

He held that job for just over two years. During his tenure, he devoted considerable effort to overhauling the nation’s nuclear posture, developing the policy of “flexible response” that was unveiled in early 1974.

Schlesinger later served as the first Secretary of Energy under President Carter, holding that position from late 1977 to mid-1979.

Over the past decade, he had served as a member of the Homeland Security Advisory Council, a consultant to the Defense Department, and a member of the Defense Policy Board.

Schlesinger’s death was announced on the same day that the Air Force is expected to reveal what disciplinary measures will be taken as a result of an investigation into a cheating scandal at Malmstrom Air Force Base, Mont., that involves 92 missile officers.